...
A couple months ago I synced a totally stock 0.8 release binary from early 2013 all the way to the current tip, with no special configuration or anything-- it just worked (though very slowly). This test lets you easily be absolutely confident that there have been no triggered hardforks in the last decade. (Latent hardforks, though, can only be found with careful code review-- certainly there have been no intentional ones since Satoshi, maybe never)
Testing older than 0.8 is harder because really old binaries don't work right on current systems and building the old code with a modern operating systems also doesn't work right. Satoshi also made an incompatible P2P change in 0.2.10 with a time delay that didn't activate until 2012-ish, so any node older than that requires modification or a custom gateway to get connected to more modern nodes.
Did you test older versions till you found one that worked or do you just know that they will not?
I'm not motivated to go install & update an old 32 bit OS just to run the old client but it would be an interesting experiment if you did not do it.
You are WAY more informed on this then me, but if 0.8 can sync, if I can can get it running from what I can see 0.6.3 should also, but it may not work properly on a modern OS. Eliminating the security implications running it on 32 bit Windows XP should work.
Just wondering, not that important.
-Dave